This invention relates generally to antennas and in particular to a compact, phasable, multioctave, high efficiency, spiral mode antenna.
There have always been numerous civilian, scientific and military requirements for a generic wideband high efficiency and low profile antenna element which can be mounted close to a ground plane. Some, but not all, of these requirements have been met with the designs of previous antennas. The history of these antenna elements can be traced back to the conical log spiral antenna. This antenna consists of two conducting sheets on a dielectric cone; the conducting sheets are fed at the cone apex with the energy traveling down the cone towards its base. The active (radiating) region of the cone is the point at which the phase of the wave traveling down the cone changes by approximately 360 degrees around the circumference of the cone. In this region a circularly polarized, backward-traveling wave is launched (passing the cone apex), having a polarization opposite to that of the element winding direction, i.e. if a right-hand wave travels down the cone, the radiated wave is left circularly polarized. If the element is a self-conjugate antenna, the conducting and non-conducting areas are equal and the two areas will be precisely interchanged under a physical 90 degree rotation.
Erickson and Fisher (Reference 1) improved upon the log spiral in a design for an element utilized in a decametric-wavelength (15-110 MHZ or 2.7-20 meters) phased-array radio telescope by replacing the balanced conducting sheets (which would present construction and wind-loading difficulties for an element designed to operate at meter wavelengths with 3 wires, i.e., the edges were defined by wires (2 wires, 1 for each edge), with a third wire located along the centerline of each surface. They also realized that the element could be operated below its cut-off frequency (the frequency at which the circumference at the base of the element was approximately 1 wavelength), albeit at reduced efficiency, by resistively terminating the element windings, at the base of the element, in the characteristic impedance of the element. The two wire-defined "surfaces" were fed through a balun (balanced-to-unbalanced transformer) from coaxial cable. Another opposed pair of winding wires between the two surfaces was electrically disconnected. Arrays of 15 elements each could be phased to a desired direction simply by electronically switching the balun to the appropriate 6 out of 8 element windings, thereby changing the phase of each element in 45-degree increments. Important conclusions they drew from precise and exhaustive measurements were: (1) the half-power beamwidth was about 100 degrees, centered on the zenith; (2) the element efficiency was within 1 to 3 dB of that of a reference dipole antenna; (3) the element phasing did indeed change by 45 degrees per rotation step; (4) cross-polarization varied from less than 5% at frequencies below 50 MHZ to 20% at 110 MHZ; and (5) the element retained its high efficiency even down to frequencies for which the radiating region was close to the ground. Conclusion (5) is implicit in their results but is not explicitly stated in their analysis. However, it is extremely important in considering how well an active region will radiate, and maintain its impedance, when it is located very close to a ground plane. The height of their log spiral antenna was 7.2 meters.
A broadband but linearly-polarized antenna (Reference 2) constructed with wire elements outlining current sheet surfaces also displayed efficient operation at frequencies for which the active radiating region was very close to a ground plane. However, it had no phasing capability.
An advance in log spiral antennas was made by Wang and Tripp (References 3-5) who designed a planar log spiral antenna which could be operated at a very small fraction of a wavelength above a ground plane, thereby resulting in a low-profile element suitable for a variety of civilian and military applications. In commercial literature describing the antenna element, they refer to a compact version of the element which, however, has only limited bandwidth.